The present invention generally relates to an arrangement for irradiation of products with charged particles.
By existing technology, there are presently three main approaches of sterilising. The first approach is by heating in an autoclave. This method can only be used for heat resistive materials. Since many products are not heat resistive, they are not treatable in an autoclave.
Another approach is to expose the product to poisonous gas. This poisonous gas is normally ethylene oxide, which is poisonous, carcinogenic and explosive. The use of this gas is surrounded by extensive security regulations. Among other things, the products have to be ventilated during a very long time period in order to decrease the rest level of gas in the material to approved levels. The products are packed in a semi-permeable package and ventilating of gas takes place through this layer. Bacterial cultivation is requested before the products are allowed to be delivered. Together, all this results in that a normal treatment time becomes 7-10 days, transports excluded. Rest levels of gas in the material are considered as very dangerous and the tendency is that the allowed limits are continuously lowered.
The third approach is to use ionising radiation. A great advantage with such methods is that they are considered as very safe, and for confirmed absorbed radiation doses above 25 kGy, no bacterial cultivation has to be performed before delivery. Furthermore, the products may be packed before the sterilisation, since the radiation penetrates the package materials. One type of ionising radiation used is gamma radiation from cobalt sources. These radiation sources are generally very strong, in the order of magnitude of 1 MCi, which requires strong radiation shields, e.g. concrete walls with a thickness of 2 m. The penetration ability is very good, but the exposure time is very long, sometimes up to several days.
Another type of ionising radiation is charged particles from accelerators, preferably electrons. They have a more limited penetration depth, but is generally much easier to handle. Known technology uses 10 MeV electrons to achieve a penetration depth in the materials which is large enough. This calls for large accelerators and even in such cases, the radiation shield may consist of up to 2 m of concrete. In order to be able to use such methods, practically and economically, large separate central plants are demanded. Contract sterilising is the normal proceeding for plants of ionising radiation, which means that the sterilising is performed separate from the production, which in turn gives rise to long storage times and transport costs. The investments for such plants are in the order of magnitude of 30-60 million Swedish kronor.
An alternative manner is to irradiate the products with charged particles of a lower energy, preferably electrons, which gives a lower penetration depth. In order to be able to use this method in practice, by use of known technology, which only irradiates the products from one side, the products have to be turned and run another time through the sterilising device to achieve a sufficient penetration depth. This normally gives rise to internal logistic problems and risks for mishandling, unless double arrangements are used after each other.
One way to irradiate an object from different directions is known from INP Novosibirsk (c.f. the U.S. Pat. No. 4,121,086). In this concept, the electron beam from an accelerator is deflected into two alternative beam paths besides the undeflected beam path, which three beam paths in the end impinges on the radiation target in one and the same spot, but from three different directions. The deflection is performed by a deflection magnet and redeflection of the deflected beams is performed by two redeflection magnets. However, this radiation source only operates with three discrete beam paths with individually scanned beam. Such a radiation device is mainly suited for radiation of products with circular cross section, or fluids transported through the irradiation area in circular pipes.
In the U.S. Pat. No. 4,201,920, an irradiation arrangement for irradiation of products from two sides with a scanning electron beam is disclosed. The radiation target is asymmetrically arranged in the area scanned by the electron beam and electrons not impinging directly on the radiation target are deflected to impinge on the back side of the radiation target. The pole pieces of the electromagnet are adapted dependent on the shape and size of the radiation target, to give a homogeneous irradiation. However, this equipment has a number of severe disadvantages.
A first disadvantage is that one has to modify the geometrical shape of the pole pieces for irradiation of products with different shape or size in order to, according to the description, achieve an optimally efficient irradiation. This implies a costly and time consuming pole change when changing the products to be irradiated. If instead the same pole pieces are kept, the patent does not disclose anything about how a control of the scanning could make the use of the beam time more efficient.
Furthermore, the electrons which are redeflected are moving along a longer geometrical path, which means other focusing properties for the redeflected electron beam as compared with the directly impinging electron beam, when they impinge on the products to be irradiated. How such irradiation inhomogenities are to be compensated for is not discussed in the document.
A third disadvantage with the device in the U.S. Pat. No. 4,201,920 arises at irradiation of products which do not continuously occupies the full available radiation sector, e.g. for irradiation of products of an uneven shape or when the products are separated by an interspace. This is the normal case during production of medical disposable products. In these cases, at least a part of the beams will pass the radiation area without being absorbed. These will instead be incident back towards the products and may cause incorrect and inhomogeneous dose distribution. Furthermore, this effect is not equal for the two different sides of the product.
To use an irradiation arrangement efficiently, the products to be irradiated are normally transported in and out from the radiation sector during operation. This is performed by means of any conveying system or assembly line system through the irradiation arrangement. A usual problem is that products are stuck or moving on the conveyor belt. This is particularly true for small, irregular and flabby packages. The most common way of conveying is to let the products lie on a conveyor belt, which passes them into the irradiation arrangement, through the radiation sector and out from the arrangement. The path of the conveyor belt has to be bent in order to be able to efficiently protect for secondary X-ray radiation, i.e. pass through a so-called labyrinth. The risk for that the products are moving at the conveyor belt or are stuck within the irradiation arrangement is large by such technical solutions. The result is varying uncontrolled radiation doses and risk for fire, respectively, since a power of above 6 kW is used. If an internal radiation dose measurement is used, all electronics is rapidly destroyed by the ionising radiation and has to be replaced periodically. If the products, considering the economical efficiency, are packed close together, the risk for overlapping, shadowing and halts increases with an unacceptable quality as the result.
In order to overcome the above described disadvantages and to provide an irradiation arrangement which is simple and small enough for being installed directly in a production line, the present invention presents a solution. The invention provides a device for double-sided irradiation of the products by electrons with a relatively low energy (1-10 MeV), and preferably between 1, 5 and 2, 5 MeV, which penetrates goods with a thickness less than 1 g/cm2. The device comprises controllable means, which causes the particle beam to scan over the surface of the product from two sides with controllable focusing properties. The scanned particle beams are preferably incident towards the product in an angle close to 90 degrees and particles, not absorbed by the product or in the vacuum windows surrounding the products, impinge on a particle stopper, supplied for this purpose. The controllable means preferably comprises a focusing lens, a controllable scanning magnet for deflection of the particle beam and two redeflection magnets for bringing the particle beams back to the radiation area for the products. The scanning magnet and the focusing lens are controlled in such a manner that a homogeneous irradiation is achieved over the entire radiation sector from two opposite directions.
A conveyor device has been constructed, which allows the double-sided irradiation at the same time as it flexibly fixes the products during the transport through the irradiation arrangement, whereby the radiation dose to which the products are exposed may be totally controlled by the feeding velocity of the conveyor device.
Owing to that a lower particle energy and a double-sided irradiation is used, the size of the arrangement, including the radiation shield may be made relatively small. Together with the design of the conveyor device, this results in that the arrangement may fit together with a normal production line.